(4‐Acyl‐5‐pyrazolonato)titanium Derivatives: Oligomerization, Hydrolysis, Voltammetry, and DFT Study

Twenty 4‐acyl‐5‐pyrazolonato (Q) titanium derivatives of varied nuclearity have been synthesized from Ti(OR) 4 or TiCl 4 and characterized with spectroscopic methods (IR, NMR, ESI‐MS). While Ti−(β‐diketonato) cleavage is not seen in isolated solids, Ti−O(alkoxy) (or Ti−Cl) bonds cleave upon hydrolysis, leading to several structural forms, including oligomers. Ionic Q species with no Ti, i.e., obtained after Ti−Q cleavage, are seen for some Ti−Q derivatives by ESI‐MS, which also indicates a varied nuclearity for a given species, e.g., the isolated polynuclear [Q 2 Ti‐μ‐O] n has several “ n ” values. Mononuclear Ti complexes are obtained under rigorous anhydrous conditions. The cis structures of the mononuclear species (Q T ) 2 Ti(OCH 3 ) 2 , Q T = 3‐methyl‐4‐(neopentylcarbonyl)‐1‐phenylpyrazol‐5‐onato have been analyzed with DFT methods. A trans influence is a major driving force that accounts for several sets of Ti−O bonds. One of the cis stereoisomers is 56 kcal/mol higher in energy than the other two. In contrast, all (Q T ) 2 TiCl 2 cis isomers show similar energies. Voltammetry of the mononuclear species (Q T ) 2 Ti(O n Pr) 2 and the antitumor tetranuclear compound [(Q B ) 2 Ti‐μ‐O] 4 , (Q B = 4‐benzoyl‐3‐methyl‐1‐phenylpyrazol‐5‐onato) indicate that the Ti IV is less prone to reduction to Ti III in the latter ( E p c for the Ti IV /Ti III couple is −1.71 V and −1.46 V versus Fc + /Fc, respectively). Potential antitumor compounds having a Ti/Q ratio of 1:1 do not disproportionate, unlike the equivalent acetylacetonato derivatives, and are water‐soluble. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)